This invention relates to a control method and a control apparatus for an automatic welding machine in which a welding gun is moved by a robot successively to a plurality of welding spots on a workpiece in order to carry out spot welding.
In a conventional automatic welding machine, certain steps of operation are successively performed. After a welding gun is moved by operation of a robot to a welding spot, the workpiece is clamped under pressure between electrode tips attached to the front ends of a pair of gun arms of the welding gun and then the gun is energized. The welding gun is opened after a lapse of a predetermined hold time following completion of the energization. When the opening of the welding gun is confirmed by a signal from a sensor which detects the relative displacement between the pair of gun arms of the welding gun, the robot is operated to move the welding gun to the next welding spot.
In some cases, however, one of the electrode tips of the welding gun becomes fusion-welded to the workpiece during the welding operation. With an automatic welding machine of the foregoing type, even when an electrode tip attached to one of the gun arms of the welding gun becomes fusion-welded to the workpiece, the other gun arm is free to move to open if the welding gun is operated for opening thereof. Thus, the sensor detecting the relative displacement between the two gun arms outputs the signal indicating that the welding gun is open, and the robot is therefore activated to move the welding gun. This can cause the gun arm to be subjected to undue force and/or the workpiece to be damaged since the tip is still fusion-welded to the workpiece.
As disclosed in the Japanese Unexamined Utility Model Registration Application Publication Jikkai Sho 58-9287, a welding gun fusion-weld detection apparatus can include a pair of limit switches to detect the actual return of each of the gun arms of the welding gun to their respective open positions. Whether or not there is fusion weld of one of the gun arm electrode tips to the workpiece is determined by signals from these limit switches.
By making use of the technology disclosed in the abovementioned publication, it is possible to arrange the welding machine so that the robot may be operated when a pair of the gun arms of the welding gun are both returned to their respective open positions. However, it should be noted that it takes about 80 milli-seconds or so for the gun arms to return to their open positions after the welding gun is given an instruction to open. This much time required to confirm that the welding gun is opened each time the welding at a welding spot is completed can accumulate a great loss of time that can not be disregarded when the number of welding spots is of considerable magnitude.
The present invention has for its object to provide a control method and a control apparatus for an automatic welding machine, wherein the time needed to detect whether or not there is a fusion weld of an electrode tip of the gun arm of the welding gun to the workpiece is reduced as much as possible so as to contribute to improvement of the work efficiency.